1. Field of the Invention
The present invention relates in general to a single chip with a piezoelectric crystal device integrated on a monolithic integrated circuit, and more particularly to a single-chip radio structure with a piezoelectric crystal device such as a surface acoustic wave resonator, precise reference frequency generator and other passive devices integrated on a monolithic integrated circuit and a method of fabricating the same, in which a silicon substrate, a thick or thin piezoelectric crystal on silicon (TPCS) and a metal layer are provided to implement a single-chip transmission/reception system for a portable terminal.
2. Description of the Prior Art
In portable communication systems which have recently been studied, there has been of more importance a technique for fabricating devices which are low in power consumption, small in size and low in cost under the condition that they satisfy all specifications on reception sensitivity, channel selection, etc. Due to the recent trend where devices become smaller in size through developments in microelectronic fabrication technology, the above aims have been accomplished to a certain degree with respect to intergrated circuits composed of a number of transistors. However, passive devices such as a crystal resonator, filter, inductor, etc. are discrete components which are still large in volume. As a result, such devices are the main obstacles in the miniaturation of a transceiver.
Generally, a clock generator or a reference frequency generator for communication equipment comprises a hybrid circuit consisting of a crystal resonator, oscillator and control circuit, which may be constructed in any one of various manners such as Colpitts, Hartley and Clapp. A typical Colpitts-type oscillator is shown in FIG. 1, herein.
In a conventional oscillator shown in FIG. 1, a crystal resonator X10 is operated based on a characteristic of a quartz crystal which is very stable in mechanical resonance frequency. The quartz crystal is of a thin membrane shape and the resonance frequency thereof is proportional to inverse of the thickness thereof. Noticeably, because a resonance frequency of 100 MHZ corresponds to a very thin thickness of about several tens .mu.m, it is impossible to obtain the higher resonance frequency.
The low resonance frequency of the crystal may be made higher by using a frequency multiplier composed of an inductor L19, resistor R20 and capacitor C21 in FIG. 1 or a so-called phase locked loop (PLL). Alternatively, a surface acoustic wave resonator shown in FIG. 2 may be replaced for the conventional crystal to raise a basic mode resonance frequency. However, such devices are conventionally used as external devices because they are difficult to be implemented on silicon integrated circuits. In particular, the surface acoustic wave resonator is difficult to be integrated. Many studies have been made since the 1970's of using, for a UHF-band reference signal generator, a surface acoustic wave resonator which is capable of obtaining a high frequency and has an advantage of ease in mass production, instead of conventional means based on a high-frequency resonance mode of a crystal and a frequency multiplier (see: Acoustic Surface Wave Resonator Devices, U.S. Pat. No. 3,886,504, May 20, 1974).
FIGS. 2 and 3 show a conventional 2-terminal surface acoustic wave resonator. This surface acoustic wave resonator is to use a phenomenon where a resonance occurs as an elastic wave or acoustic wave is produced on a piezoelectric substrate 51 and then blocked by acoustic wave reflectors 56 and 57. In this resonator, the basic principle of the resonace and equivalent circuit model are the same as those of a conventional quartz crystal resonator.
But, the surface acoustic wave resonator utilizes a surface acoustic wave component which mechanically resonates along the surface of a crystal on which various electrodes 52-57 are laid, whereas the conventional crystal resonator utilizes a resonance mode of a bulk acoustic wave which mechanically resonates in the direction of a thickness of a thin crystal.
In the 2-terminal surface acoustic wave resonator shown in FIGS. 2 and 3, when a variation of a voltage signal across lead wires 62 and 64 in FIG. 2 is applied to transducer electrodes 52 and 54 formed on the surface of a crystal, a fine mechanical displacement is formed on and propagated along the crystal surface due to properties of a piezoelectric material to produce a surface acoustic wave. The produced surface acoustic wave is detected by a transducer composed of electrodes 53 and 55, which transduces the mechanical resonance into an electrical signal in the reverse procedure of the generation of surface waves.
In such a surface acoustic wave device, a bulk acoustic wave component may be generated, reflect from the bottom of the crystal bordered on a metal plate 60 and return to the transducer electrodes 53 and 55 formed on the surface of the crystal. This bulk acoustic wave component becomes a factor of deteriorating characteristics of the device. In order to suppress such an effect of the bulk acoustic wave component on the characteristics of the device, an acoustic absorption material such as epoxy may be applied on the bottom of the crystal to absorb a part of the wave component arriving at the bottom and irregularly reflect the remainder.
A resonance frequency of the surface acoustic wave resonator is determined depending on a distance or pitch between two electrodes just adjacent respectively to the reflectors 56 and 57, twice which is a wavelength of a resonance mode.
Accordingly, the maximum possible resonance frequency of the surface acoustic wave resonator is determined according to a line width in a semiconductor manufacturing process. Typically, a lithography process with a resolution of up to 1 .mu.m is used to obtain a basic mode resonance frequency higher than 1 GHz.
Noticeably, the above-mentioned surface acoustic wave resonator or conventional crystal resonator is provided with one type of substrate 51 composed of only a piezoelectric crystal. For this reason, a hybrid circuit must be provided on a printed circuit board (PCB) to connect the surface acoustic wave resonator to other integrated circuits formed on silicon or GaAs substrates. In this case, signals must be passed through a bonding wire 61 and lead wires 62, 63, 64 and 65 for connection with the chip circuits, resulting in increases in occupied area and PCB manufacturing cost as compared with a single-chip integrated circuit structure.
As a result, studies have recently been made of integrating surface acoustic wave devices with silicon or gallium-arsenide circuits on a single chip. Such studies may generally be classified into a method of forming a surface acoustic wave device by depositing a piezoelectric material such as ZnO or AlN on a silicon substrate (see: ZnO Films on {110}-Cut &lt;100&gt;-Propagating GaAs Substrates for Surface Acoustic Wave Device Applications, IEEE Trans. Ultrason. Ferroelec. Freq. Cont., vol. 42. pp. 351-361, 1995) and a method of providing a surface acoustic wave device in an integrated circuit by bonding a thick crystal or LiNbO.sub.3 crystal on silicon (see: Integrated Circuit including a Surface Acoustic Wave Transformer and a Balanced Mixer, U.S. Pat. No. 5,265,267, 1993).
However, in the deposition method, a frequency stability of the deposited ZnO or AlN with respect to variations in temperature and time is inferior to that of a piezoelectric single crystal. For this reason, the deposition method is not suitable for a reference frequency generator of a communication system with a strict standard.
Generally, in a piezoelectric crystal, the speed of a surface acoustic wave varies on the order of 20 to 30 ppm with respect to a temperature variation of 0.degree. to 50.degree.. Further, the piezoelectric crystal has a long-term stability wherein a resonance frequency varies on the order of 1 ppm for one year, which is on an improved trend. In this connection, in the case where the piezoelectric crystal is used as a substrate of a surface acoustic wave resonator, it will offer sufficient frequency stability and precision, as is well known in the art.
Hence, in order to maintain a frequency stability at a degree satisfying the communication system standard and increase an integration level of a transceiver circuit without depending on a frequency multiplier generating undesirable higher harmonics, there is required a method of bonding a piezoelectric crystal on silicon to make a piezoelectric crystal/silicon structure and forming a surface acoustic wave device and other passive devices on the piezoelectric crystal/silicon structure.
On the other hand, in the second method of bonding a thick crystal or LiNbO.sub.3 crystal on silicon, the associated fabrication method is not specified and the entire device is large in thickness because the piezoelectric crystal is not adjusted in thickness. Further, in the case where a surface acoustic wave device is formed on the piezoelectric crystal, it is difficult to be connected to an integrated circuit. Moreover, it is hard to provide a single-chip transmitter/receiver or radio structure with even passive devices such as an inductor, transmission line, etc., integrated therein.